Conventionally, rubber articles, such as belts of a radial tire, body plies of a carcass, and belt members for use in various industries, have used brass-plated steel wires or steel cords of a plurality of brass-plated steel wires stranded together, which are coated with rubber. In this way, they have gained a reinforcement effect on the rubber thereof. To achieve such a reinforcement effect, an adequate adhesion must be ensured between the brass-plated steel wire and the rubber coating thereon. For example, in the curing process of tire manufacture, the steel cord in contact with the rubber is heated to form an adhesion layer through a reaction of sulfur in the rubber with copper in the brass plating. Therefore the steel cords for use in rubber articles are required to have high performance (adhesion performance) of quickly and reliably forming an adhesion layer.
In a conventional method proposed for improving the adhesion performance between the brass-plated steel wire and the rubber, a noncrystalline portion consisting of crystal grains of 20 nm or less in grain size is formed on the surface side of the crystalline portion of the brass plating layer consisting of crystal grains of over 20 nm in grain size. And this noncrystalline portion promotes a quick progress of adhesive reaction between the brass-plated steel wire and the rubber (see Reference 1, for instance).
Reference 1: Japanese Unexamined Patent Application Publication No. 2006-283270